1. Field of the Invention
The present invention relates to conjugation-stabilized (poly)peptide and protein compositions and formulations, and to methods of making and using same.
2. Description of the Related Art
The use of polypeptides and proteins for the systemic treatment of certain diseases is now well accepted in medical practice. The role that the peptides play in replacement therapy is so important that many research activities are being directed towards the synthesis of large quantities by recombinant DNA technology. Many of these polypeptides are endogenous molecules which are very potent and specific in eliciting their biological actions.
A major factor limiting the usefulness of these substances for their intended application is that they are easily metabolized by plasma proteases when given parenterally. The oral route of administration of these substances is even more problematic because in addition to proteolysis in the stomach, the high acidity of the stomach destroys them before they reach their intended target tissue. Polypeptides and protein fragments, produced by the action of gastric and pancreatic enzymes, are cleaved by exo and endopeptidases in the intestinal brush border membrane to yield di-and tripeptides, and even if proteolysis by pancreatic enzymes is avoided, polypeptides are subject to degradation by brush border peptidases. Any of the given peptides that survive passage through the stomach are further subjected to metabolism in the intestinal mucosa where a penetration barrier prevents entry into the cells.
In spite of these obstacles, there is substantial evidence in the literature to suggest that nutritional and pharmaceutical proteins are absorbed through the intestinal mucosa. On the other hand, nutritional and drug (poly)peptides are absorbed by specific peptide transporters in the intestinal mucosa cells. These findings indicate that properly formulated (poly)peptides and proteins may be administered by the oral route, with retention of sufficient biological activity for their intended use. If, however, it were possible to modify these peptides so that their physiological activities were maintained totally, or at least to a significant degree, and at the same time stabilize them against proteolytic enzymes and enhance their penetration capability through the intestinal mucosa, then it would be possible to utilize them properly for their intended purpose. The product so obtained would offer advantages in that more efficient absorption would result, with the concomitant ability to use lower doses to elicit the optimum therapeutic effect.
The problems associated with oral or parenteral administration of proteins are well known in the pharmaceutical industry, and various strategies are being used in attempts to solve them. These strategies include incorporation of penetration enhancers, such as the salicylates, lipid-bile salt-mixed micelles, glycerides, and acylcarnitines, but these frequently are found to cause serious local toxicity problems, such as local irritation and toxicity, complete abrasion of the epithelial layer and inflammation of tissue. These problems arise because enhancers are usually co-administered with the peptide product and leakages from the dosage form often occur. Other strategies to improve oral delivery include mixing the peptides with protease inhibitors, such as aprotinin, soybean trypsin inhibitor, and amastatin, in an attempt to limit degradation of the administered therapeutic agent. Unfortunately these protease inhibitors are not selective, and endogenous proteins are also inhibited. This effect is undesirable.
Enhanced penetration of peptides across mucosal membranes has also been pursued by modifying the physicochemical properties of candidate drugs. Results indicate that simply raising lipophilicity is not sufficient to increase paracellular transport. Indeed it has been suggested that cleaving the peptide-water hydrogen bonds is the main energy barrier to overcome in obtaining peptide diffusion across membranes (Conradi, R. A., Hilgers, A. R., Ho, N. F. H., and Burton, P. S., "The influence of peptide structure on transport across Caco-2 cells", Pharm. Res., 8, 1453-1460, (1991)). Protein stabilization has been described by several authors. Abuchowski and Davis ("Soluble polymers-Enzyme adducts", In: Enzymes as Drugs, Eds. Holcenberg and Roberts, J. Wiley and Sons, New York, N.Y., (1981)) disclosed various methods of derivatization of enzymes to provide water soluble, non-immunogenic, in vivo stabilized products.
A great deal of work dealing with protein stabilization has been published. Abuchowski and Davis disclose various ways of conjugating enzymes with polymeric materials (Ibid). More specifically, these polymers are dextrans, polyvinyl pyrrolidones, glycopeptides, polyethylene glycol and polyamino acids. The resulting conjugated polypeptides are reported to retain their biological activities and solubility in water for parenteral applications. The same authors, in U.S. Pat. No. 4,179,337, disclose that polyethylene glycol rendered proteins soluble and non-immunogenic when coupled to such proteins. These polymeric materials, however, did not contain fragments suited for intestinal mucosa binding, nor did they contain any moieties that would facilitate or enhance membrane penetration. While these conjugates were water-soluble, they were not intended for oral administration.
Meisner et al., U.S. Pat. No. 4,585,754, teaches that proteins may be stabilized by conjugating them with chondroitin sulfates. Products of this combination are usually polyanionic, very hydrophilic, and lack cell penetration capability. They are usually not intended for oral administration.
Mill et al., U.S. Pat. No. 4,003,792, teaches that certain acidic polysaccharides, such as pectin, algesic acid, hyaluronic acid and carrageenan, can be coupled to proteins to produce both soluble and insoluble products. Such polysaccharides are polyanionic, derived from food plants. They lack cell penetration capability and are usually not intended for oral administration.
In Pharmacological Research Communication 14, 11-120 (1982), Boccu et al. disclosed that polyethylene glycol could be linked to a protein such as superoxide dismutase ("SOD"). The resulting conjugated product showed increased stability against denaturation and enzymatic digestion. The polymers did not contain moieties that are necessary for membrane interaction and thus suffer from the same problems as noted above in that they are not suitable for oral administration.
Other techniques of stabilizing peptide and protein drugs in which proteinaceous drug substances are conjugated with relatively low molecular weight compounds such as aminolethicin, fatty acids, vitamin B.sub.12, and glycosides, are revealed in the following articles: R. Igarishi et al., "Proceed. Intern Symp. Control. Rel. Bioact. Materials, 17, 366, (1990); T. Taniguchi et al. Ibid 19, 104, (1992); G. J. Russel-Jones, Ibid, 19, 102, (1992); M. Baudys et al., Ibid, 19, 210, (1992). The modifying compounds are not polymers and accordingly do not contain moieties necessary to impart both the solubility and membrane affinity necessary for bioavailability following oral as well as parenteral administration. Many of these preparations lack oral bioavailability.
Another approach which has been taken to lengthen the in vivo duration of action of proteinaceous substances is the technique of encapsulation. M. Saffan et al., in Science, 223, 1081, (1986) teaches the encapsulation of proteinaceous drugs in an azopolymer film for oral administration. The film is reported to survive digestion in the stomach but is degraded by microflora in the large intestine, where the encapsulated protein is released. The technique utilizes a physical mixture and does not facilitate the absorption of released protein across the membrane.
Ecanow, U.S. Pat. No. 4,963,367, teaches that physiologically active compounds, including proteins, can be encapsulated by a coacervative-derived film and the finished product can be suitable for transmucosal administration. Other formulations of the same invention may be administered by inhalation, oral, parenteral and transdermal routes. These approaches do not provide intact stability against acidity and proteolytic enzymes of the gastrointestinal tract, the property as desired for oral delivery.
Another approach taken to stabilize protein drugs for oral as well as parenteral administration involves entrapment of the therapeutic agent in liposomes. A review of this technique is found in Y. W. Chien, "New Drug Delivery Systems", Marcel Dekker, New York, N.Y., 1992. Liposome-protein complexes are physical mixtures; their administration gives erratic and unpredictable results. Undesirable accumulation of the protein component in certain organs has been reported, in the use of such liposome-protein complexes. In addition to these factors, there are additional drawbacks associated with the use of liposomes, such as cost, difficult manufacturing processes requiring complex lypophilization cycles, and solvent incompatibilities. Moreover, altered biodistribution and antigenicity issues have been raised as limiting factors in the development of clinically useful liposomal formulations.
The use of "proteinoids" has been described recently (Santiago, N., Milstein, S. J., Rivera, T., Garcia, E., Chang., T. C., Baughman, R. A., and Bucher, D., "Oral Immunization of Rats with Influenza Virus M Protein (M1) Microspheres", Abstract #A 221, Proc. Int. Symp. Control Rel. Bioac. Mater., 19, 116 (1992)). Oral delivery of several classes of therapeutics has been reported using this system, which encapsulates the drug of interest in a polymeric sheath composed of highly branched amino acids. As is the case with liposomes, the drugs are not chemically bound to the proteinoid sphere, and leakage of drug out of the dosage form components is possible.
A peptide which has been the focus of much synthesis work, and efforts to improve its administration and bioassimilation, is insulin.
The use of insulin as a treatment for diabetes dates back to 1922, when Banting et al. ("Pancreatic Extracts in the Treatment of Diabetes Mellitus," Can. Med. Assoc. J., 12, 141-146 (1922)) showed that the active extract from the pancreas had therapeutic effects in diabetic dogs. Treatment of a diabetic patient in that same year with pancreatic extracts resulted in a dramatic, life-saving clinical improvement. A course of daily injections of insulin is required for extended recovery.
The insulin molecule consists of two chains of amino acids linked by disulfide bonds; the molecular weight of insulin is around 6,000. The .beta.-cells of the pancreatic islets secrete a single chain precursor of insulin, known as proinsulin. Proteolysis of proinsulin results in removal of four basic amino acids (numbers 31, 32, 64 and 65 in the proinsulin chain: Arg, Arg, Lys, Arg respectively) and the connecting ("C") peptide. In the resulting two-chain insulin molecule, the A chain has glycine at the amino terminus, and the B chain has phenylalanine at the amino terminus.
Insulin may exist as a monomer, dimer or a hexamer formed from three of the dimers. The hexamer is coordinated with two Zn.sup.2+ atoms. Biological activity resides in the monomer. Although until recently bovine and porcine insulin were used almost exclusively to treat diabetes in humans, numerous variations in insulin between species are known. Porcine insulin is most similar to human insulin, from which it differs only in having an alanine rather than threonine residue at the B-chain C-terminus. Despite these differences most mammalian insulin has comparable specific activity. Until recently animal extracts provided all insulin used for treatment of the disease. The advent of recombinant technology allows commercial scale manufacture of human insulin (e.g., Humulin.TM. insulin, commercially available from Eli Lilly and Company, Indianapolis, Ind.).
Although insulin has now been used for more than 70 years as a treatment for diabetes, few studies of its formulation stability appeared until two recent publications (Brange, J., Langkjaer, L., Havelund, S., and Volund, A., "Chemical stability of insulin. I. Degradation during storage of pharmaceutical preparations," Pharm. Res., 9, 715-726, (1992); and Brange, J. Havelund, S., and Hougaard, P., "Chemical stability of insulin. 2. Formulation of higher molecular weight transformation products during storage of pharmaceutical preparations," Pharm. Res., 9, 727-734, (1992)). In these publications, the authors exhaustively describe chemical stability of several insulin preparations under varied temperature and pH conditions. Earlier reports focused almost entirely on biological potency as a measure of insulin formulation stability. However the advent of several new and powerful analytical techniques--disc electrophoresis, size exclusion chromatography, and HPLC--allows a detailed examination of insulin's chemical stability profile. Early chemical studies on insulin stability were difficult because the recrystallized insulin under examination was found to be no more than 80-90% pure. More recently monocomponent, high-purity insulin has become available. This monocomponent insulin contains impurities at levels undetectable by current analysis techniques.
Formulated insulin is prone to numerous types of degradation. Nonenzymatic deamidiation occurs when a side-chain amide group from a glutaminyl or asparaginyl residue is hydrolyzed to a free carboxylic acid. There are six possible sites for such deamidiation in insulin: Gln.sup.A5, Gln.sup.A15, Asn.sup.A18, Asn.sup.A21, Asn.sup.B3, and Gln.sup.B4. Published reports suggest that the three Asn residues are most susceptible to such reactions.
Brange et al. (ibid) reported that in acidic conditions insulin is rapidly degraded by extensive deamidation at Asn.sup.A21. In contrast, in neutral formulations deamidation takes place at Asn.sup.B3 at a much slower rate, independent of insulin concentration and species of origin of the insulin. However, temperature and formulation type play an important role in determining the rate of hydrolysis at B3. For example, hydrolysis at B3 is minimal if the insulin is crystalline as opposed to amorphous. Apparently the reduced flexibility (tertiary structure) in the crystalline form slows the reaction rate. Stabilizing the tertiary structure by incorporating phenol into neutral formulations results in reduced rates of deamidation.
In addition to hydrolytic degradation products in insulin formulations, high molecular weight transformation products are also formed. Brange et al. showed by size exclusion chromatography that the main products formed on storage of insulin formulations between 4.degree. and 45.degree. C. are covalent insulin dimers. In formulations containing protamine, covalent insulin protamine products are also formed. The rate of formulation of insulin-dimer and insulin-protamine products is affected significantly by temperature. For human or porcine insulin, (regular N1 preparation) time to formation of 1% high molecular weight products is decreased from 154 months to 1.7 months at 37.degree. C. compared to 4.degree. C. For zinc suspension preparations of porcine insulin, the same transformation would require 357 months at 4.degree. C. but only 0.6 months at 37.degree. C.
These types of degradation in insulin may be of great significance to diabetic subjects. Although the formation of high molecular weight products is generally slower than the formation of hydrolytic (chemical) degradation products described earlier, the implications may be more serious. There is significant evidence that the incidence of immunological responses to insulin may result from the presence of covalent aggregates of insulin (Robbins, D. C. Cooper, S. M. Fineberg, S. E., and Mead, P. M., "Antibodies to covalent aggregates of insulin in blood of insulin-using diabetic patients", Diabetes, 36, 838-841, (1987); Maislos, M., Mead, P. M., Gaynor, D. H., and Robbins, D. C., "The source of the circulating aggregate of insulin in type I diabetic patients is therapeutic insulin", J. Clin. Invest., 7.7, 717-723. (1986); and Ratner R. E., Phillips, T. M., and Steiner, M., "Persistent cutaneous insulin allergy resulting from high molecular weight insulin aggregates", Diabetes, 39, 728-733, (1990)). As many as 30% of diabetic subjects receiving insulin show specific antibodies to covalent insulin dimers. At a level as low as 2% it was reported that the presence of covalent insulin dimers generated a highly significant response in lymphocyte stimulation in allergic patients. Responses were not significant when dimer content was in the range 0.3-0.6%. As a result it is recommended that the level of covalent insulin dimers present in formulation be kept below 1% to avoid clinical manifestations.
Several insulin formulations are commercially available; although stability has been improved to the extent that it is no longer necessary to refrigerate all formulations, there remains a need for insulin formulations with enhanced stability. A modified insulin which is not prone to formation of high molecular weight products would be a substantial advance in the pharmaceutical and medical arts, and modifications providing this stability (and in addition providing the possibility of oral availability of insulin) would make a significant contribution to the management of diabetes.
In addition to the in vivo usage of polypeptides and proteins as therapeutic agents, polypeptides and proteins also find substantial and increasing use in diagnostic reagent applications. In many such applications, polypeptides and proteins are utilized in solution environments wherein they are susceptible to thermal and enzymic degradation of (poly)peptides and proteins such a enzymes, peptide and protein hormones, antibodies, enzyme-protein conjugates used for immunoassay, antibody-hapten conjugates, viral proteins such as those used in a large number of assay methodologies for the diagnosis or screening of diseases such as AIDS, hepatitis, and rubella, peptide and protein growth factors used for example in tissue culture, enzymes used in clinical chemistry, and insoluble enzymes such as those used in the food industry. As a further specific example, alkaline phosphatase is widely utilized as a reagent in kits used for the colorimetric detection of antibody or antigen in biological fluids. Although such enzyme is commercially available in various forms, including free enzyme and antibody conjugates, its storage stability and solution often is limited. As a result, alkaline phosphatase conjugates are frequently freeze-dried, and additives such as bovine serum albumin and Tween 20 are used to extend the stability of the enzyme preparations. Such approaches, while advantageous in some instances to enhance the resistance to degradation of the polypeptide and protein agents, have various shortcomings which limit their general applicability.